DESCRIPTION(Adapted from the applicant's abstract): Tuberculosis (TB) is the most common fatal infectious disease in the world, and remains a threat to health in the United States. While improved case finding and access to treatment have reduced the incidence of TB in the United States, further improvements in TB control and therapy depend on better understanding of the pathogenesis of TB. Macrophages are essential for defense against infections, and are central to the pathogenesis of TB. When macrophages encounter most bacteria, they phagocytose and kill them. In contrast, macrophages phagocytose, but do not kill M. tuberculosis, even when they are stimulated with IFN gamma. Recent experiments in the PI's laboratory reveal that one means that M. tuberculosis uses to evade killing by macrophages is to block the signal transduction pathway initiated by interferon gamma. The PI has found that infection of macrophages with M. tuberculosis blocks several macrophage responses to IFN gamma, and has found that this disruption of signalling reduces transcriptional activation of IFN gamma-responsive genes at a distal step in the signalling pathway. M. tuberculosis infection of macrophages causes release of one or more soluble factors that inhibit IFN gamma signaling in uninfected macrophages. TGF-beta, IL-4, IL-6, IL-10, and prostaglandin E2 cannot account for this phenomenon. The PI proposes to identify the component of M. tuberculosis that initiates the inhibition of IFN gamma signaling. One hypothesis to be tested is that infection of macrophages by M. tuberculosis induces a repressor that binds specific DNA elements in the promoter region of interferon gamma-responsive genes. Finally, the PI will purify the soluble factor present in the conditioned medium from infected cells that inhibits interferon gamma signaling in uninfected macrophages. The proposed experiments will enhance the understanding of the pathogenesis of tuberculosis, and will provide insight essential for developing effective approaches to enhancing the protective immune response to M. tuberculosis.